Catechol-O-methyltransferase (COMT) characteristics and activity were studied using radioenzymatic techniques in placentas from normal and high-risk patients at term. The affinity of the catechol estrogen 20-hydroxyestrone (2OHE1), Km = 5 microM, for the enzyme was found to be at least 90-fold higher than that of the catecholamines norepinephrine, epinephrine, and dopamine (450, 490, and 850 microM, respectively). The product formed after incubation of placental cytosol with tritiated 20HE1 was identified as being exclusively (3H) 2-methoxyestrone, by use of high-performance liquid chromatography. Placental COMT activity after normal spontaneous delivery was no different from that obtained after elective cesarean section (270 +/- 20 versus 275 +/- 27, expressed as mean +/- SE activity nanomoles of 2-methoxyestrone formed per hour/mg protein). Placental COMT activity at term in women with hypertension (toxemia [T] or chronic hypertension [CHBP]) was significantly lower than in control subjects (284 +/- 27 versus 183 +/- 26; P less than 0.05). No significant differences in enzyme activity were found between T and CHBP (175 +/- 37 versus 210 +/- 32, P = NS). There were also no differences in COMT activity of diabetic classes A-R (White classification), fetal distress (with or without acidosis), and controls. The possible interference of antihypertensive drugs used by the patients in the study on COMT activity was assessed. Incubations of healthy placental cytosol with hydralazine, methyldopa, and Mg++ in their estimated plasma therapeutic concentrations had no effect on placental COMT activity. The present study suggests that placental COMT activity is low in patients with hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)